When to declare protected fields in abstract class cpp

Question

I feel very uncomfortable with the concept of abstract classes, and I'd be happy if someone could make it a bit clearer. I do understand that it's also a matter of implementation and personal style, but I'd like to have at least some guidelines to think about when trying to solve a problem.

I thought that abstract classes only represent a type of behavior that their derived classes will inherit. since we cannot instantiate an object from an abstract class type, so if they have a c'tor or any data fields, they should be in protected mode. I've encountered a piece of code today, where the abstract class had a public constructor and it made me even more confused. I have few questions about this:

1. the only unique thing about abstract classes is that they have pure virtual functions? it's the only thing that prevents me from instantiating object from this class

2. on what occasions do I want to declare protected fields in an abstract class? when is it useful to add protected constructor? when is a public constructor needed?

I see many different uses of abstract classes, but sometimes I think about totally different solution, which most of the times is much more complicated and cumbersome than the one presented.

Thank you for your time and attention everyone.

Answer 1

Use abstract classes when you want to provide a base implementation for something, leave some parts of the implementation up to the child classes, but make it mandatory to implement the bits and pieces that are missing.

the only unique thing about abstract classes is that they have pure virtual functions?

A class is abstract because some parts of it (methods) are abstract too (have no implementation).

However, you can declare a class as abstract even if it has no abstract method. That is you want to prevent people from instantiating it. (you could in that case also declare the constructor protected)

on what occasions do I want to declare protected fields in an abstract class?

On the same occasion you would declare protected fields for any class: some properties which may be used by child classes

when is it useful to add protected constructor?

When you want to prevent instantiation of objects of a class. If the class is abstract, there is no additional benefit as it will not be intantiable anyway.

when is a public constructor needed?

For abstract classes, it is not. But that is valid. That constructor will then be used by child classes to pass parameters from their own constructors. In the case of abstract classes it may be declared protected without any side effect.

Kindly note that I am no C++ expert, but those questions are valid in any OOP language.